1. Field of the Invention
The invention relates to an image stabilization system for diagnostic eye examination.
2. Description of Prior Art
In a number of diagnostic procedures it is necessary to examine detail within the eye using optical instruments such as ophthalmoscopes, biomicroscopes, or specular microscopes. The patient's eye, however, is never actually stationary for a period of time long enough to perform a detailed examination. For low magnification applications, such as the indirect ophthalmoscope, eye motion is of little consequence. However, when using instruments having higher magnification, eye motion becomes increasingly troublesome as the magnification is increased. An example is the visualization of the endothelial cells of the cornea.
Voluntary eye motions can be controlled by a cooperative patient. However, there are frequent motions which are involuntary and are not controllable by even the most cooperative subject. These small, sudden motions or microsaccades occur on the order of 70 times per minute. The tremors may extend over only 5 or 6 minutes of an arc; which in fact corresponds to one or more endothelial cell diameters. Furthermore, any changes in the longitudinal position of the cornea results in defocus of the image.
If, for example, the eye makes a sudden rotational movement of half a degree, the cornea will move laterally approximately 0.1 mm. Endothelial cells are approximately 0.025 mm in width, therefore the motion of the cornea in this example would be equivalent to four cell widths. In a microscope system designed to view the cells at high magnification, such motion of the image makes detailed study of the cells difficult if not impossible. Present methods involve photography of the image using a flash lamp to stop the motion. While this technique produces satisfactory photographic results, it is often desirable to examine the endothelial cells visually in detail. A method for stabilizing the image is therefore highly desirable.
One previously used technique for decreasing the motion of the eye during a procedure known as specular microscopy has been to contact the cornea with a so-called dipping cone objective. In this technique the outer-most element of the objective has a flat, polished glass surface. When the objective is in contact with the cornea, rotational motion of the eye is inhibited (but not entirely eliminated).
The major disadvantage of this technique is that any motions of the eye relative to the dipping cone can cause epithelial cells to be abraded off of the cornea. While the cornea is anesthetized, the pressure against the eye is felt by the patient and is uncomfortable. Continued use of the dipping cone for thorough studies of the cornea can lead to a roughened epithelial surface. And while the body repairs this tissue in a matter of hours, the degraded optical quality of the exterior surface of the cornea decreases the quality of the images which can be obtained.
Diagnostic contact lenses are known in the art. Such lenses generally are hand-held in contact with the cornea, and as a result, do not move with the eye as it rotates. Their primary function is to permit the fundus of the eye and certain other interior regions of the eye to be viewed at low magnification, for example, through a biomicroscope. Such lenses do not reduce the effect of eye rotation on the image to any substantial extent.